The present invention relates generally to the power regulating and electrostatographic printing arts. More particularly, the invention concerns an rms voltage controller for ensuring constant fuser temperature by controlling the power dissipation of a fixed load. While the specific intention is to control a fuser, the same techniques may be applied to any high current load where line voltage variations cause flicker.
In a preferred form, a controller in accordance with the invention is advantageously employed to control the rms voltage supplied to a fusing apparatus of an electrostatographic printing machine. In the process of xerography, an exemplary form of electrostatographic printing, heat is applied to permanently affix powder toner images to a variety of support surfaces, such as individual copy sheets. This process of applying heat is conventionally referred to as fusing and is carried out by a fusing apparatus, or simply a fuser. A resistance element, such as a lamp, is typically employed to generate the heat necessary for the fusing process. To maintain a consistent level of copy quality, it is necessary to maintain the temperature of the fuser within a critical tolerance range. If the fuser temperature is too low, fusing of the powder images may be incomplete, producing smeared or incompletely copied final images. Fuser temperatures which are too high raise the likelihood that the copy sheets may scorch or bum. The sources to which printing machines are connected, a separate isolated voltage source typically 115 volts AC, exhibit inevitable variations in the line voltage supplied. A separate isolated circuit is usually employed to prevent lighting flicker due to the load current variations generated by the electrostatographic printing machine""s fuser controller.
An objective to the present invention is to eliminate the light flickering problem so that electrostatographic printing machine can be connected to a common circuit thereby avoiding the cost and problems associated with providing a separate circuit for the electrostatographic printing machine.
In recognition of these voltage fluctuations, a variety of regulating devices have been heretofore developed. For instance, it is known in the prior art to control the power input to the fuser in response to voltage levels across the fuser heat source. U.S. Pat. No. 3,881,085 to Traister, discloses a fuser control circuit in which a switching means, such as a silicon controlled rectifier is triggered to interrupt power to the fuser heating source when a preset level of line voltage is detected across the heating element. Separate R/C circuitry is used to set and reset an amplifier to selectively inhibit the silicon controlled rectifier and thus interrupt power supply to the heating element. Another prior art control system is shown in U.S. Pat. No. 3,735,092 to Traister. A thermistor senses changes in the fuser temperature, providing a signal which controls a switching amplifier. When a normal operating temperature is attained in the fuser, the switching amplifier is triggered to a non-conducting state which opens a switch to interrupt power to the fuser heating element. Another known class of regulating device seeks to maintain a constant power input to the fuser. In U.S. Pat. No. 3,961,236 to Rodek et al., for example, constant power regulation is sought by monitoring both the voltage across the fuser load and the current therethrough.
A summation of the detected load voltage and current provides an approximation of the power consumption which is utilized to control the power input to the fuser. To effect the desired control, a triac is selectively gated, i.e. triggered on and off, to inhibit the supply of power from the source to the fuser circuitry, the triggering being effected at zero crossing points of the supply voltage waveform for predetermined numbers of half cycles. Another illustrative circuit for regulating the power applied to a load by controlling the number of cycles of supplied voltage is shown in U.S. Pat. No. 3,579,096 to Buchanan. U.S. Pat. No. 4,223,207 to Chow discloses a circuit for controlling the power supplied to a load by varying the duty cycle of the AC signal supplied to the load. Other known control systems have been developed to regulate rms voltage across a fuser element. Since it may generally be assumed that the resistance of the fuser element will not change appreciably, it follows that control of the rms voltage across the load will effectively control the power dissipated thereby. In one such controller, a digital signal equivalent of a sample of the fuser input voltage is supplied to a processor. In response to the digitized signal, the processor selectively gates the input voltage source across the fuser heating element in accordance with a plurality of gate activation rates stored in a register associated with the processor. The foregoing controllers are either costly or do not optimally deliver accurate, precise, control of power supplied to the load. Moreover the problem with lighting flicker still remains, if these devices are connected to a common circuit.
In accordance with the invention, there is provided a control system for delivering a constant fuser temperature while reducing lighting flicker to an acceptably low level despite variations in the line voltage. In general, this is effected by a circuit which employs closed loop feedback to control the temperature of the fuser. This is accomplished by a circuit and method which functionally provides a continuous solution to the equation which describes the relationship between the temperature of the fuser and the temperature of a desired control setpoint. Briefly, the solution of this equation is obtained by monitoring, i.e. sampling, the temperature of the fuser, subtracting the desired control temperature, and then integrating the difference over time. The resultant time integral is used to control the operating point of a Pulse Width Modulated (PWM) power controller. The integrator has a time constant sufficiently long and the power controller operates at a frequency sufficiently high so as to render lighting flicker imperceptible. The control circuit of this invention is particularly advantageous in controlling the rms voltage across a radiant fuser lamp in an electrostatographic printing machine. In such an application, the circuitry preferably includes a microprocessor which controls a pulse width modulated power chain to regulate the input line voltage across the fuser heating element. In this preferred form, the fuser temperature is converted into a representative signal. The microprocessor is programmed to act as an integrator that continuously averages the difference between this sampled temperature and the predetermined control temperature. When this continuous summation equals a fixed reference, predetermined in accordance with the system equation, the microprocessor changes the pulse width modulator duty ratio, controlling the voltage applied to the load. Special care is taken to limit the rate at which the processor changes the PWM duty ratio, thereby eliminating lighting flicker.